USO0RE43188E
(19) United States (12) Reissued Patent
(10) Patent Number: US RE43,188 E (45) Date of Reissued Patent: Feb. 14, 2012
Robertsson et a]. (54)
(56)
METHOD AND SYSTEM FOR REDUCING EFFECTS OF SEA SURFACE GHOST CONTAMINATION IN SEISMIC DATA
References Cited U.S. PATENT DOCUMENTS 2,757,356 A 7/1956 Haggerty
(75) Inventors: Johan Robertsson, Grantchester (GB); Julian Edward Kragh, Finching?eld (GB); James Edward Martin, Hovik
(Continued) FOREIGN PATENT DOCUMENTS
(GB) (73) Assignee: Schlumberger Technology
2 090 407 A
7/1982
(Continued)
Corporation, Sugar Land, TX (U S) OTHER PUBLICATIONS
(21) App1.No.: 12/264,784 (22)
Filed:
Pierson, W J and MoskoWitZ, L A Proposed Spectral From for Fully Developed Wind Seas Based on the Similarity Theory of S A
Nov. 4, 2008
Kitaigorodskii Journal of Geophysical Research, vol. 69, No. 24, Dec. 1964, pp. 5181-5190. Hasselmann, D E, Dunckel, M and Ewing J A Directional Wave Spectra Observed during JONSWAP 1973 Journal of Physical Oceanography, vol. 10, 1980, pp. 1264-1280.
Related US. Patent Documents
Reissue of:
(64)
Patent No.: Issued:
Appl. No.: PCT Filed: PCT No.:
6,775,618 Aug. 10, 2004 09/936,863
(Continued)
Mar. 21, 2000 Primary Examiner * Michael Nghiem
PCT/GB00/01074
Assistant Examiner * Toan M Le
8371 (0X1), (2), (4) Date:
Nov. 30, 2001
(57) ABSTRACT An improved de-ghosting method and system that utiliZes
PCT Pub. No.: WO00/57207
PCT Pub. Date: Sep. 28, 2000
multi-component marine seismic data recorded in a ?uid medium. The method makes use of tWo types of data: pressure data that represents the pressure in the ?uid medium, such as sea Water, at a number of locations; and vertical particle motion data that represents the vertical particle motion of the acoustic energy propagating in the ?uid medium at a number
US. Applications: (62) Division of application No. 11/501,195, ?led on Aug. 8, 2006, noW Pat. No. Re. 41,656.
(30)
Foreign Application Priority Data
Mar. 22, 1999
of locations Within the same spatial area as the pressure data.
(GB) .................................... .. 9906456
The vertical particle motion data can be in various forms, for
(51)
Int. Cl. G01 V 1/00 G01 V 1/38
example, velocity, pressure gradient, displacement, or accel eration. A spatial ?lter is designed so as to be effective at
(2006.01) (2006.01)
(52)
US. Cl. .......................................... .. 702/14; 367/24
(58)
Field of Classi?cation Search .................. .. 367/15,
separating up and doWn propagating acoustic energy over substantially the entire range of non-horizontal incidence angles in the ?uid medium. The spatial ?lter is applied to either the vertical particle motion data or to the pressure data, and then combined With the other data to generate pressure data that has its up and doWn propagating components sepa rated.
367/20*21, 24, 87489; 702/14, 142, 17418, 702/81, 84, 127, 1374138, 1424143, 1504153, 702/182*183,189*191,195,197; 181/101, 181/108,110*113, 122; 703/5, 9410 See application ?le for complete search history.
8 Claims, 13 Drawing Sheets 206
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US RE43,188 E Page 2 U.S. PATENT DOCUMENTS 7/1973 Greene, Jr. 3,747,055
4,222,266 4,486,865 4,979,150 5,051,961 5,365,492 5,524,100 5,581,514 5,621,700 5,696,734 5,754,492 5,850,922 6,101,448 6,493,636
E>D
9/ 1980 Theodoulou 12/ 1984 Ruehle 12/ 1990 Barr *
9/1991
Corrigan et al. .............. .. 367/24
11/1994 Dragoset, Jr. 6/1996
* *
Paffenholz .................... .. 367/24
12/1996 Moldoveanu et al. 4/ 1997
Moldoveanu .... ..
*** 12/1997 Corrigan *
5/1998
Starr ............................. .. 367/24
12/1998 Fraser 8/2000
Ikelle et al. ................... .. 702/17
12/2002
DeKok .......................... .. 702/17
FOREIGN PATENT DOCUMENTS GB GB WO
2 333 364 A 2 341 680 A 97/44685 A1
7/1999 3/2000 11/1997
OTHER PUBLICATIONS Robertsson, J O A A numerical free-surface condition for elastic/
viscoelastic ?nite-difference modeling in the presence of topo graphy Geophysics, vol. 61, No. 6, Nov.-Dec. 1996, pp. 1921-1934. Robertsson, J O A, Blanch, J O and Symes, WWViscoelastic ?nitei difference modeling Geophysics, vol. 59, No. 9, Sep. 1994, pp. 1444 1456.
Barr, F J and Sanders, J I Attenuation of Water-Column Reverbera tions Using Pressure and Velocity Detectors in aWater-Bottom Cable
Annual Meeting of Society EXpl. Geophys, Jan. 1989, XP 000672198, pp. 653-656. Amundsen, L, Seacrest, B G and Arntsen, B Extraction of the normal
component of the particle velocity from marine pressure data Geophysics, vol. 60, No. 1, Jan-Feb. 1995, pp. 212-222. Schneider, W A, Lamer, K L., Burg, J P and Backus, M M A new dataiprocessing technique for the elimination of ghost arrivals on re?ection seismograms Geophysics, vol. 29, No. 5, Oct. 1964, pp. 783-805. White, J E Plane Waves Seismic Waves: radiation, transmission and
attenuation, McGraW-Hill, 1965, chapter 2, pp. 14-77. * cited by examiner
US. Patent
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US RE43,188 E 1
2
METHOD AND SYSTEM FOR REDUCING EFFECTS OF SEA SURFACE GHOST CONTAMINATION IN SEISMIC DATA
for attenuating water-column re?ections’, (hereinafter “Barr (1990)”). For all practical purposes, this was previously described by White, J. E., in a 1965 article entitled ‘Seismic waves: radiation, transmission and attenuation’, McGraw
Hill (hereinafter “White (1965)”). However, this technique is
Matter enclosed in heavy brackets [ ] appears in the original patent but forms no part of this reissue speci?ca
not effective when the angle of incidence is away from verti cal.Also, this technique does not completely correct for wide
tion; matter printed in italics indicates the additions made by reissue.
angle scattering and the complex re?ections from rough sea surfaces. Additionally, its is believed that the OBC techniques described have not been used successfully in a ?uid medium, such as with data gathered with towed streamers.
More than one reissue applications have been filedfor the reissue ofU.S. Pat. No. 6, 775,618, where this application Ser.
SUMMARY OF THE INVENTION
No. 12/264, 784, filed on Nov. 4, 2008 is a divisional reissue
Thus, it is an object of the present invention to provide a
application ofa co-pending U.S. reissue application Ser. No. 11/501,195, filed on Jun. 8, 2006, for the reissue ofU.S. Pat.
method of de-ghosting which improves attenuation of noise from substantially all non-horizontal angles of incidence.
No. 6, 775,618, which was a nationalphase application, Ser.
No. O9/936,863,?led on Sep. 18, 2001 ofan international application, Ser. No. PCT/GBOO/O1074, ?led on Mar 21, 2000, which claims priority ofa GB application, S/N GB 9906456, ?led on Mar. 22, 1999; and where U.S. reissue
20
downward propagating noise from substantially all non-hori Zontal angles of incidence.
application Ser No. 11/501,195 was issued as US. Pat. No.
Also, it is an object of the present invention to provide a method of de-ghosting which is not critically dependent on
Re. 41,656 on Sep. 7, 2010. FIELD OF THE INVENTION
It is an object of the present invention to provide a method of de-ghosting of seismic measurements made in a ?uid medium which improves attenuation of the ghost as well as
25
knowledge about the properties of the surrounding ?uid medium as well as hydrophone/geophone calibration and
coupling compensation.
The present invention relates to the ?eld of reducing the effects of sea-surface ghost re?ections in seismic data. In
particular, the invention relates an improved de-ghosting method that utilises measurements or estimates of multi component marine seismic data recorded in a ?uid medium.
30
Also, it is an object of the present invention to provide a method of de-ghosting whose exact implementation is robust and can be implemented e?iciently. According to the invention, a method is described for sea
surface ghost correction through the application of spatial BACKGROUND OF THE INVENTION
Removing the ghost re?ections from seismic data is for
?lters to the case of marine seismic data acquired in a ?uid 35
many experimental con?gurations equivalent to up/down wave ?eld separation of the recorded data. In such con?gu rations the down-going part of the wave?eld represents the
ghost and the up-going wave?eld represents the desired sig nal. Exact ?lters for up/down separation of multi-component
measurements taken along a marine towed streamer, for
example. New streamer designs are currently under develop 40
wave?eld measurements in Ocean Bottom Cable (OBC) con
?gurations have been derived by Amundsen and Ikelle, and ar described in UK. Patent Application Number 9800741 .2. An example of such a ?lter corresponding to de-ghosting of pressure data at a frequency of 50 HZ for a sea?oor with
medium. Using, for example, either typical towed streamer or vertical cable geometries. Preferably, both pressure and ver tical velocity measurements are acquired along the streamer. The invention takes advantage of non-conventional velocity ment and are expected to become commercially available in the near future. For example, the Defence Evaluation and
Research Agency (DERA), based in Dorset, U.K., claim to have successfully built such a streamer for high frequency 45
sonar applications. According to an alternative embodiment, the invention is
P-velocity of 2000 m/s, S-velocity of 500 m/s and density of
also applicable to seismic data obtained with con?gurations
1800 kg/m3 is shown in FIG. 2. At this frequency, the maxi mum horizontal wavenumber for P-waves right below the sea?oor is k:0.157 m_l, whereas it is k:0.628 m‘1 for
of multiple conventional streamers. Here, the ?lters make use of vertical pressure gradient measurements, as opposed to velocity measurements. According to the invention, an esti mate of the vertical pressure gradient can be obtained from over/under twin streamer data, or more generally from streamer data acquired by a plurality of streamers where the streamers are spatially deployed in a manner analogous to that
S-waves. Notice the pole and the kink due to a Zero in the ?lter
50
at these two wavenumbers, making approximations neces sary for robust ?lter implementations. FIG. 3 shows approxi mations co the ?lter. These ?lters are only good at wavenum
bers smaller than the wavenumber where the pole occurs.
Hence, energy with low apparent velocities (for instance
55
S-waves or Scholte waves at the sea?oor) will not be treated
properly. Moreover, since they do not have a complex part, evanescent waves will also not be treated properly.
The OBC de-ghosting ?lters have been shown to work very
well on synthetic data. However, apart from the di?iculty with poles and Zeros at critical wave numbers, they also require knowledge about the properties of the immediate sub-bottom locations as well as hydrophone/geophone calibration and
coupling compensation. A normal incidence approximation to the de-ghosting ?l
described in UK Patent Application Number 98200496, by Robertsson, entitled ‘Seismic detection apparatus and related method” ?led in 1998 (hereinafter “Robertsson (1998)”). For example, three streamers can be used, forming a triangular shape cross-section along their length. Vertical pressure gra dient data can also be obtained from pressure gradient mea
60
suring devices. According to the invention, the ?lters fully account for the rough sea perturbed ghost, showing improvement over other techniques based on normal incidence approximations (see e.g., White (1965)), which have been applied to data recorded
65 at the sea ?oor.
ters for data acquired at the sea ?oor was described by Barr, F.
Advantageously, according to preferred embodiments of
J. in US. Pat. No. 4,979,150, issued 1990, entitled ‘System
the invention, the results are not sensitive to streamer depth,
US RE43,188 E 3
4
allowing the streamer(s) to be towed at depths below swell
FIG. 2 shows an exact pressure de-ghosting ?lter for OBC data for a sea?oor with P-velocity of 2000 m/ s, S-velocity of
noise contamination, hence opening up the acquisition weather window where shallow towed streamer data would be unusable. Local streamer accelerations will be minimised
500 m/ s and density of 1800 kg/m3; the upperpanel shows the Real part of exact ?lter; and the lower panel shows the Imagi nary part of exact ?lter; FIG. 3 shows the Real part of the exact OBC de-ghosting ?lter (in the solid line) shown in FIG. 2, the ?rst order Taylor approximation ?lter (in the plus line), and the ?rst four frac
in the deep water ?ow regime, improving resolution of the
pressure, multi-component velocity and pressure gradient measurements.
Advantageously, according to preferred embodiments of the invention, there are no ?lter poles in the data window,
tional expansion approximations ?lters (in the dash-dotted
except for seismic energy propagating horizontally at the compressional wave speed in water.
lines);
Advantageously, according to preferred embodiments of the invention, the ?lter is not critically dependent on detailed
knowledge of the physical properties of the surrounding ?uid medium.
The ?lters can be simple spatial convolutions, and with the
regular geometry of typical towed streamer acquisition the ?lters are e?icient to apply in the frequency-wavenumber (FK) domain. The ?lters can also be formulated for applica tion in other domains, such as time-space and intercept time
slowness ("c-p) According to the invention, a method of reducing the effects in seismic data of downward propagating re?ected and scattered acoustic energy travelling in a ?uid medium is pro vided. The method advantageously makes use of two types of data: pressure data, that represents the pressure in the ?uid
20
FIG. 8 is a ?ow chart illustrating some of the steps of the de-ghosting method for the combination of pressure and ver
tical velocity data to achieve separated pressure data, accord ing to a preferred embodiment of the invention; 25
medium, such as sea water, at a number of locations; and
FIG. 9 schematically illustrates an example of a data pro cessor that can be used to carry out preferred embodiments of
vertical particle motion data, that represents the vertical par ticle motion of the acoustic energy propagating in the ?uid medium at a number of locations within the same spatial area as the pressure data. The distance between the locations that
FIG. 4 illustrates the potential impact of 3D rough sea surface ghost re?ection and scattering on consistency of the seismic data waveform; FIG. 5 illustrates the potential impact of the rough sea surface ghost perturbation on time-lapse seismic data quality; FIGS. 6a-6f show various embodiments for data acquisi tion set-ups and streamer con?gurations according to pre ferred embodiments of the invention; FIG. 7 shows an exemplary two-dimensional spatial ?lter response (um/k2) for dx:6 m;
the invention; FIG. 10 shows an example of a shot record computed below a 4 m signi?cant wave height (SWH) rough sea surface,
spatial sampling criterion. The vertical particle motion data
the left panel shows pressure, and the right panel shows vertical velocity scaled by water density and the compres sional wave speed in water;
can be in various forms, for example, velocity, pressure gra dient, displacement, or acceleration. The spatial ?lter is created by calculating a number of
FIG. 11 illustrates de-ghosting results of the shot record in FIG. 10, the left panel shows results using the vertical inci dence approximation, and the right panel illustrates the exact
are represented by the pressure data and the vertical particle motion data in each case is preferably less than the Nyquist
30
35
solution;
coe?icients that are based on the velocity of sound in the ?uid
medium and the density of the ?uid medium. The spatial ?lter
FIG. 12 illustrates an example of de-ghosting results in
is designed so as to be effective at separating up and down
detail for a single trace at 330 m offset corresponding to an
propagating acoustic energy over substantially the entire range of non-horizontal incidence angles in the ?uid medium. The spatial ?lter is applied to either the vertical particle
40
motion data or to the pressure data, and then combined with the other data to generate pressure data that has its up and
down propagating components separated. The separated data
arrival angle of about 37 degrees, the upper panel shows the vertical incidence approximation, and the lower panel shows the Exact solution; and FIGS. 12a-b illustrate two possible examples of multi component streamer design.
are then processed further and analysed. Ordinarily the down going data would be analysed, but the up going data could be
DETAILED DESCRIPTION OF THE INVENTION
used instead if the sea surface was suf?ciently calm. According to an alternative embodiment, a method of
FIG. 1 is a schematic diagram showing re?ections between a sea surface (S), sea ?oor (W) and a target re?ector (T). Various events that will be recorded in the seismogram are shown and are labelled according to the series of interfaces they are re?ected at. The stars indicate the seismic source and the arrowheads indicate the direction of propagation at the receiver. Events ending with ‘S’ were last re?ected at the rough sea surface and are called receiver ghost events. Down going sea-surface ghost re?ections are an undesirable source
reducing the effects of downward propagating re?ected and scattered acoustic energy travelling in a ?uid medium is pro vided wherein the pressure data and vertical particle motion
50
data represent variations caused by a ?rst source event and a second source event. The source events are preferably gener
ated by ?ring a seismic source at different locations at differ
ent times, and the distance between the locations is preferably
less than the Nyquist spatial sampling criterion. The present invention is also embodied in a computer readable medium which can be used for directing an appara tus, preferably a computer, to reduce the effects in seismic data of downward propagating re?ected and scattered acous tic energy travelling in a ?uid medium as otherwise described herein.
55
of contamination, obscuring the interpretation of the desired up-going re?ections from the earth’s sub-surface. Rough seas are a source of noise in seismic data. Aside from the often-observed swell noise, further errors are intro 60
duced into the re?ection events by ghost re?ection and scat tering from the rough sea surface. The rough sea perturbed ghost events introduce errors that are signi?cant for time
lapse seismic surveying and the reliable acquisition of repeat
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows examples of simple seismic ray paths for primary events, and ghosts that are last re?ected from the
rough sea-surface;
65
able data for stratigraphic inversion. The effect of the rough sea is to perturb the amplitude and arrival time of the sea surface re?ection ghost and add a
scattering coda, or tail, to the ghost impulse. The impulse
US RE43,188 E 5
6
response can be calculated by ?nite difference or Kirchhoff
perturbations. A method for correcting these types of error is clearly important in such a case, and With the increasing
methods (for example) from a scattering surface Which rep resents statistically typical rough sea surfaces. For example, a directional form of the Pierson-MoskoWitZ spectrum described by Pierson, W. J. and MoskoWitZ, L., 1964 ‘A
requirement for higher quality, loW noise-?oor data, correc tion for the rough sea ghost becomes necessary even in mod est sea states.
proposed Spectral Form for Fully Developed Wind Seas
Equation (1) gives the frequency domain expression for a preferred ?lter relating the up-going pressure ?eld, p” (x), to the total pressure, p(x), and vertical particle velocity, vZ(x).
Based on the Similarity Theory of S. A. Kitaigorodskii’ J.
Geo. Res., 69, 24, 5181-5190, (hereinafter “Pierson and MoskoWitZ (1964)”), and Hasselmann, D. E., Dunckel, M. and Ewing, J. A., 1980 ‘Directional Wave Spectra Observed
During JONSWAP 1973’, J. Phys. Oceanography, v10, 1264 1280, (hereinafter “Hasselmann et al, (1980)”). Both the Wind’s speed and direction de?ne the spectra. The Signi?cant Wave Height (“SWH”) is the subjective peak to trough Wave
Where lg is the vertical Wavenumber for compressional Waves in the Water, p is the density of Water and * denotes spatial convolution.
amplitude, and is about equal to 4 times the RMS Wave height. Different realisations are obtained by multiplying the 2D
surface spectrum by Gaussian random complex numbers. FIG. 4 shoWs an example of rough sea impulses along a 400 In 2D line (e.g. streamer) computed under a 2 m SWH 3D rough sea surface. The streamer shape affects the details of
The vertical Wavenumber is calculated from kZ2:k2 —k,€2 for tWo-dimensional survey geometries, or k22:k2—kX2—ky2 for three-dimensional survey geometries, With k2:u)2/c2, Where c 20
the impulses, and in this example the streamer is straight and horizontal. FIG. 4 shoWs, from top to bottom: The ghost
Water. The regular sampling of typical toWed streamer data alloWs lg to be calculated e?iciently in the FK domain. FIG.
Wavelet (White trough) arrival time, the ghost Wavelet maxi mum amplitude, a section through the rough sea realisation
above the streamer, and the computed rough sea impulses. The black peak is the upWard travelling Wave, Which is unper turbed; the White trough is the sea ghost re?ected from the rough sea surface. The latter part of the Wavelet at each receiver is the scattering coda from increasingly more distant parts of the surface. Notice that the amplitude and arrival time
7 shoWs an example of the ?lter response, (1)/k2 for dx:6 m 25
30
ghost perturbations change fairly sloWly With offset. The Wavelengths in the sea surface, Which are 100-200 In for 2-4
m SWH seas, and the amplitude perturbations are governed 35
diffraction coda appear as quasi-random noise folloWing the
ghost pulse. The rough sea perturbations cause a partial ?ll and a shift of
the ghost notch in the frequency domain. They also add a
(the ?lter is normalised for the display to an arbitrary value). In?nite gain poles occur When lg is Zero. This corresponds to
energy propagating horizontally (at the compressional Wave speed in Water). For toWed streamer data, there is little signal
arrival time perturbations are governed by the dominant by the curvature of the sea surface Which has an RMS radius of about 80 m and is fairly independent of sea state. The
is the compressional Wave speed in the Water and k,C is the horiZontal Wavenumber for compressional Waves in the
40
small ripple to the spectrum, Which amounts to 1-2 dB of error for typical sea states. In the post stack domain this translates
energy With this apparent velocity, any noise present in the data With this apparent velocity should be ?ltered out prior to the ?lter application, or, the ?lter should be tapered at the poles prior to application to avoid ampli?cation of the noise.
The traditional ?lter (White (1965), Barr, (1990)) is equa tion (2): (2) By comparison to equation (1 ), We see that this is a normal incidence approximation, Which occurs When k,C is Zero. This is implemented as a simple scaling of the vertical velocity trace folloWed by its addition to the pressure trace. Equation (1) can also be formulated in terms of the vertical
pressure gradient (dp(x)/dZ). The vertical pressure gradient is
to an error in the signal that is about —20 dB for a 2 m SWH
proportional to the vertical acceleration:
sea.
FIG. 5 shoWs an example of how such an error can be 45
signi?cant for time-lapse surveys. The panel on the top left shoWs a post-stack time-migrated synthetic ?nite difference
Integrating in the frequency domain through division of in),
and substituting in equation (1) gives:
seismic section. The top middle panel shoWs the same data
but after simulating production in the oil reservoir by shifting the oil Water contact by 6 m and introducing a 6 m partial
50
depletion Zone above this. The small difference is just notice able on the black leg of the re?ection to the right of the fault just beloW 2 s tWo-Way travel-time. The panel on the right (top) shoWs the difference betWeen these tWo sections multi plied by a factor of 10. This is the ideal seismic response from
55
the time-lapse anomaly. The left and middle bottom panels shoW the same seismic sections, but rough sea perturbations for a 2 m SWH (as described above) have been added to the raW data before processing. Note that different rough sea effects are added to each model to represent the different seas at the time of acquisition. The difference obtainedbetWeen the tWo sections
is shoWn on the bottom right panel (again multiplied by a factor of 10). The errors in the re?ector amplitude and phase (caused by the rough sea perturbations) introduce noise of similar amplitude to the true seismic time-lapse response. To a great extent, the true response is masked by these rough sea
FIGS. 6a-6f shoW various embodiments for data acquisi tion set-ups and streamer con?gurations according to pre ferred embodiments of the invention. FIG. 6a shoWs a seismic vessel 120 toWing a seismic source 110 and a seismic
60
65
streamer 118. The sea surface is shoWn by reference number 112. In this example, the depth of streamer 118 is about 60 meters, hoWever those of skill in the art Will recognise that a much shalloWer depth Would ordinarily be used such as 6-10 meters. The dashed arroWs 122a-d shoW paths of seismic energy from source 110. ArroW 122a shoWs the initial doWn going seismic energy. ArroW 122b shoWs a portion of the seismic energy that is transmitted through the sea ?oor 114. ArroW 122c shoWs an up-going re?ection. ArroW 122d shoWs
a cloWn-going ghost re?ected from the surface. According to
US RE43,188 E 8
7
The processing described herein is preferably performed
the invention, the doWn-going rough sea receiver ghost 122d can be removed from the seismic data.
on a data processor con?gured to process large amounts of
FIGS. 6b-6f shoW greater detail of acquisition set-ups and streamer con?gurations, according to the invention. FIG. 6b
data. For example, FIG. 9 illustrates one possible con?gura tion for such a data processor. The data processor typically consists of one or more central processing units 350, main
shoWs a multi-component streamer 124. The streamer 124
memory 352, communications or I/O modules 354, graphics devices 356, a ?oating point accelerator 358, and mass stor
comprises multiple hydrophones (measuring pressure) 126a, 126b, and 126c, and multiple 3C geophones (measuring par
age devices such as tapes and discs 360. It Will be understood by those skilled in the art that tapes and discs 360 are com puter-readable media that can contain programs used to direct the data processor to carry out the processing described herein. FIG. 10 shoWs a shot record example, computed under a 4
ticle velocity in three directions x, y, and Z) 128a, 128b, and 128c. The spacing betWeen the hydrophones 126a and 126b, and betWeen geophones 128a and 128b is shoWn to be less
than 12 meters. Additionally, the preferred spacing in relation to the frequencies of interest is discussed in greater detail beloW. FIG. 6c shoWs a streamer 130 that comprises multiple
m Signi?cant Wave Height (SWH) sea and using the ?nite difference method described by Robertsson, J. O. A., Blanch, J. O. and Symes, W. W., 1994 ‘V1scoelastic ?nite-difference
hyrdophones 132a, 132b, and 132c, and multiple pressure gradient measuring devices 134a, 134b, and 134c. The spac
modelling’ Geophysics, 59, 1444-1456 (hereinafter “Rob
ing betWeen the hydrophones 132a and 132b, and betWeen pressure gradient measuring devices 134a and 134b is shoWn to be less than 12 meters. FIG. 6d shoWs a multi-streamer con?guration that com
ertsson et al. (1994)”) and Robertsson, J. O. A., 1996 ‘A Numerical Fret-Surface Condition for Elastic/V1scoelastic 20
prises hydrophone streamers 140a and 140b. The streamers
(1996)”). The streamer depth in this example is 60 m. The left panel shoWs the pressure response and the right panel shoWs the vertical velocity response scaled by the Water density and
comprise multiple hyrdophones 142a, 142b, and 142c in the case ofstreamer 140a, and 142d, 142e, and 142fin the case of
streamer 140b. The spacing betWeen the hydrophones is
Finite-difference modelling in the Presence of Topography’, Geophysics, 61, 6, 1921 -1934 (hereinafter “Robertsson
25
the compressional Wave speed in Water. A paint source 50 HZ
shoWn to be less than 12 meters. The separation betWeen steamers 140a and 140b in the example shoWn in FIG. 6d is
this example. The choice of streamer depth alloWs a clear
less then 2 meters. Although the preferred separation is less than 2 meters, greater separations are contemplated as being
separation of the doWnWard travelling ghost from the upWard travelling re?ection energy for visual clarity of the de-ghost
Within the scope of the invention. FIG. 6e shoWs a cross sectional vieW of a dual streamer arrangement. FIG. 6f shoWs
Ricker Wavelet Was used and the streamer depth Was 60 m in
30
a multi-streamer con?guration comprising three hydrophone streamers 140a, 140b, and 140c.
Adequate spatial sampling of the Wave?eld is highly pre ferred for the successful application of the de-ghosting ?lters. For typical toWed streamer marine data, a spatial sampling interval of 12 m is adequate for all incidence angles. HoWever, to accurately spatially sample all frequencies up to 125 HZ (for all incidence angles), a spatial sampling interval of 6.25 meters is preferred. These spacings are determined according to the Nyquist spatial sampling criterion. Note that if all incidence angles are not required, a coarser spacing than described above can be used. The ?lters can be applied equally to both group formed or point receiver data. FIG. 8 is a ?oW chart illustrating some of the steps of the de-ghosting method for the combination of pressure and ver
are evident. 35
the result using the exact solution. The exact solution shoWs a consistent response over all offsets, Whereas the normal inci 40
45
spatial ?lter coef?cients are calculated. The coef?cients are 50
55
the exact solution. Not only does the exact solution provide a
superior result in terms of the de-gho sting, but also in terms of
amplitude preservation of the signal re?ectionithe upper panel shoWs loss of signal amplitude after the de-ghosting. 60
The ?lters described herein are applicable to, for example, measurements of both pres sure and vertical velocity along the
65
streamer. Currently, hoWever, only pressure measurements are commercially available. Therefore, engineering of streamer sections that are capable of commercially measuring vertical velocity is preferred in order to implement the ?lters. FIGS. 13a-b illustrate tWo possible examples of multi
step 214 the ?ltered vertical particle motion data are added to pressure data 212 to give the doWnWard propagating compo nent of the separated pressure data. Alternatively, in step 216 the ?ltered vertical particle motion data are subtracted from
pressure data 212 to give the upWard propagating component of the separated pressure data. Finally, in step 218 the upWard component is further processes and analysed.
close to its apparent velocity. The exact ?lter is tapered before application such that it is has near unity response for frequen cies and Wavenumbers corresponding to apparent velocities of 1500 m/ s and greater. The Weak event just beloW the signal re?ection is a re?ection from the side absorbing boundary of the model. It is upWard travelling and hence untouched by the ?lter. FIG. 12 shoWs details of the de-ghosted results for a single
a 37 degree incidence angle. The upper panel shoWs the normal incidence approximation, and the loWer panel shoWs
pressure data 212 are received, typically stored as time domain traces on a magnetic tape or disk. In step 210, the vertical particle motion data 208 are convolved in With the
spatial ?lter to yield ?ltered vertical particle motion data. In
dence approximation starts to break doWn at incident angles greater than about 20 degrees, and shoWs a poorer result at the near offsets. Note that the direct Wave is not ampli?ed by the exact ?lter application even though the poles of the ?lter lie
trace from FIG. 11. The trace offset is 330 m corresponding to
vertical particle motion measuring devices, and the spatial aperture of the ?lter), the density of the ?uid medium 206, and the speed of the compressional Wave in the ?uid medium (or velocity of sound) 204. Vertical particle motion data 208 and
FIG. 11 shoWs the results of de-ghosting the shot record shoWn in FIG. 10. The left panel shoWs the result using the
normal incidence approximation and the right panel shoWs
tical velocity data to achieve separated pressure data, accord ing to a preferred embodiment of the invention. In step 202, preferably dependent on the cbaracteristics of the acquisition parameters 203 (such as the temporal sample interval of the pressure and particle motion data, the spatial separation of the
ing results. The trace spacing on the plot is 24 m. A single re?ection and its associated ghost are shoWn, along With the direct Wave travelling in the Water layer. Perturbations in the ghost Wavelet and scattering noise from the rough sea surface
component streamer design. FIG. 13a shoWs a coincident
pressure and single 3-component geophone. In this design,
US RE43,188 E 9
10
the 3-component geophone is perfectly decoupled from the
calculating a plurality of spatial ?lter coe?icients based in part on the velocity of sound in the ?uid medium, the density of the ?uid medium and a plurality of acquisition parameters,
streamer. FIG. 13b shows a coincident pressure and tWin
3-component geophones. In this design, one of the 3-compo nent geophones is decoupled from the streamer, the other is
thereby creating a spatial ?lter Which is designed so as to be effective at separating up and doWn propagating acoustic energy over a range of non-vertical incidence angles in the
coupled to the streamer; measurements from both are com bined to remove streamer motion from the data. In an alternative formulation, the ?lters make use of verti
?uid medium; applying the spatial ?ller to the vertical particle motion data to generate ?ltered particle motion data; combining the ?ltered particle motion data With the pressure data to generate separated pressure data, the separated pres
cal pressure gradient measurements. An estimate of vertical pressure gradient can be obtained from over/under tWin streamers (such as shoWn in FIGS. 6d and 6e) and multiple
streamers (such as shoWn in FIG. 6f) deployed in con?gura tions analogous to that described in Robertsson (1998), alloWing the ?lters to be directly applied to such data. HoW ever, for the results to remain su?iciently accurate, the
sure data having up and doWn propagating components sepa
rated; and analysing at least part of the up or doWn propagating com
streamers should not be vertically separated by more than 2 m
ponent of the separated pressure data,
for seismic frequencies beloW approximately 80 HZ.
and Wherein said vertical particle motion data is measured
An important advantage of multiple streamer con?gura tions such as shoWn in FIG. 6f is that their relative locations are less crucial than for over/under tWin streamer geometries, Where the tWo streamers are preferably directly above one another.
using one or more multi-component streamers or vertical
cables having receivers located substantially above the sea 20
[2. The method of claim 1 Wherein the acquisition param eters include the temporal sampling interval, the spatial sam pling interval, and the number of independent locations at
The ?lters described here are applied in 2D (along the streamer) to data modelled in 2D. The application to toWed
streamer con?gurations naturally lends itself to this imple mentation, the cross-line (streamer) sampling of the Wave ?eld being usually insu?icient for a full 3D implementation. Application of these ?lters to real data (With ghost re?ections
Which the pressure and vertical particle motion data are mea 25
sured.]
[3. The method of claim 1 Wherein the vertical particle
motion data is measured using one or more multi-component
streamers]
from 3D sea surfaces) Will give rise to residual Coors caused
by scattering of the Wave?eld from the cross-line direction. This error increases With frequency though is less than 0.5 dB in amplitude and 3.6° in phase for frequencies up to 150 HZ,
?oor]
30
[4. The method of claim 1 Wherein the vertical particle motion of the acoustic energy represented in said vertical
particle motion data is the particle velocity of the acoustic
for a 4 m SWH sea. These small residual noise levels are
energy.]
acceptable When time-lapse seismic surveys are to be con ducted.
[5. The method of claim 1 Wherein the vertical particle motion of the acoustic energy represented in said vertical particle motion data is the vertical pressure gradient of the
Invoking the principle of reciprocity, the ?lters can be
35
applied in the common receiver domain to remove the doWn
acoustic energy.]
Ward travelling source ghost. Reciprocity simply means that
[6. The method of claim 5 Wherein the pressure gradient is measured using at least tWo parallel streamer cables in close
the locations of source and receiverpairs can be interchanged,
(the ray path remaining the same) Without altering the seismic response. FIG. 1 can also be used to de?ne the source ghost if the stars are noW regarded as receivers and the direction of the arroWs is reversed, With the source noW being located at the
40
motion of the acoustic energy represented in said vertical particle motion data is the vertical displacement of the acous
tic energy.]
arroW. This application is particularly relevant for data
acquired using vertical cables, Which may be tethered, for example, to the sea ?oor, or suspended from buoys. In the case of FIG. 6a, those of skill in the art Will understand that as the
proximity and vertically offset from one another] [7. The method of claim 1 Wherein the vertical particle
45
[8. The method of claim 1 Wherein the vertical particle motion of the acoustic energy represented in said vertical particle motion data is the vertical acceleration of the acoustic
energy.]
seismic vessel 120 travels though the Water, the ?ring position
[9. The method of claim 1 Wherein the distance betWeen the
of source 110 Will change. The different positions of source
110 can be then be used to construct data in the common ?rst location and the second location and the distance receiver domain as is Well knoWn in the art. 50 betWeen the third location and the fourth location is less than
the Nyquist spatial sampling criterion]
While preferred embodiments of the invention have been
described, the descriptions and ?gures are merely illustrative
[10. The method of claim 9 Wherein the spatial area is substantially a portion of a line, and the range of non-vertical
and are not intended to limit the present invention.
What is claimed is:
[1. A method of reducing the effects in seismic data of doWnWard propagating re?ected and scattered acoustic energy travelling in a ?uid medium comprising the steps of: receiving pressure data representing at least the pressure in
55
the portion of line] [11. The method of claim 9 Wherein the spatial area is a
portion of a substantially planar region, and the range of non-vertical incidence angles include substantially all non
the ?uidmedium at a ?rst location and a second location,
the ?rst location being in close proximity to the second
60
horiZontal incidence angles] [12. A method of reducing the effects in seismic data of doWnWard propagating re?ected and scattered acoustic energy travelling in a ?uid medium comprising the steps of:
location; receiving vertical particle motion data representing at least the vertical particle motion of acoustic energy propagat ing in the ?uid medium at a third location and a fourth
location, the third location being in close proximity to the fourth location, and the ?rst, second, third and fourth locations being Within a spatial area;
incidence angles includes substantially all non-horizontal incidence angles Within a vertical plane that passes through
65
receiving pres sure data representing at least the pres sure in the ?uid medium at a ?rst location and a second location,
the ?rst location being in close proximity to the second
location;
